Chapter 3, Genetics - SL Bio

DNA

(Deoxyribonucleic acid,) the substance that codes for and determines the characteristics of an organism

Chromosome

structures that organise DNA

Genes

segments of DNA that code for particular traits

Locus

location of a gene (plural: loci)

Alleles

Different versions of a gene that code for different variations of one trait

* For example, the eye colour gene has alleles that code for different eye colours and shades
* As they are forms of one gene, they are very similar. Alleles only differ from each other by one or a few bases

Mutation

Change in a gene

Sickle Cell Anaemia

* Disease causing red blood cells to become hard, sticky, and look like a C-shaped structure called a “sickle”


* Makes haemoglobin levels become clinically low
* Caused by a mutation, specifically the 6th codon for the beta chain of haemoglobin

Genome

All the genetic information a cell, organism, or organelle has

Human genome consists of:

* 46 chromosomes (including 2 sex chromosomes)
* \~3 billion base pairs
* \~21,000 genes

Point mutations are and may involve:

Changes to a base of DNA code

* The *substitution* of a base (e.g. A**T**G becomes A**C**G)


* The *insertion* of a base (e.g. ATG becomes AT**C**G)
* The *deletion* of a base (e.g. A**T**G becomes AG)
* The *inversion* of bases (e.g. A**TG** becomes A**GT**)

Nucleoid

region in the cytoplasm where a prokaryote’s DNA is found

Prokaryote DNA

* One chromosome in the nucleoid called a “genophore”
* Circular DNA molecule (genophore)
* DNA is “naked” as the nucleoid is membrane-less

Plasmid

Additional circular DNA the prokaryote may have, not present in eukaryotes, can self-replicate

Organisation of eukaryotic chromosomes

* DNA is compacted with eight histone proteins to form a structure called a *nucleosome*
* Nucleosomes are joined by an additional histone protein (H1 histone) to form a string of *chromatosomes*
* These then coil to form a *solenoid structure* (\~6 chromatosomes per turn) which is condensed to form a *30 nm (nanometer) fibre*
* These fibres then form loops, which are compressed and folded around a protein scaffold to form *chromatin*
* Chromatin will then supercoil during cell division to form *chromosomes* that are visible (when stained) under microscope

Chromatin

Mixture of DNA, RNA, and proteins that form eukaryote chromosomes

Centromere

A constricted region on the chromosome that divides the chromosome into 2 sections (or “arms”) The shorter section is called the p arm and the longer section is called the q arm

Identifying loci (gene locations)

* The first reference is a number (or letter) which denotes the chromosome (e.g. **7**q31 refers to chromosome 7) 
* The second reference is a letter (p or q) to denote which arm the locus is positioned on (e.g. 7**q**31 is on the q arm)
* The third reference is a number corresponding to the G band location (e.g. 7q**31** is at the longitudinal position 31)

Homologous Chromosomes

Maternal and paternal chromosome pairs inherited from parents

Homologous Chromosomes share:

* The same structural features (e.g. same size, same banding patterns, same centromere positions)
* The same genes at the same loci positions (while the genes are the same, alleles may be different)

Diploid

* Cell containing 2 complete sets of chromosomes (one from each parent), for example, a zygote


* All somatic (body) cells in the organism will be diploid, with new diploid cells created via mitosis
* Diploid cells are present in most animals and many plants

Haploid

* Cells containing only one set of chromosomes


* All sex cells (gametes) in the organism will be haploid, and are derived from diploid cells via meiosis
* Haploid cells are also present in bacteria (asexual) and fungi (except when reproducing)

Diploids have…

46 chromosomes, 23 pairs

Haploids have…

23 chromosomes

Autosome

Any chromosome that is not a sex chromosome, most eukaryotes have 22 pairs of autosomes, control all traits not linked to sex

Sex Chromosome

* Chromosomes determining sex, XX in women, XY in men
* Most eukaryotes have 2 pairs


* The sex chromosomes are homologous in females (XX) but are **not** homologous in males (XY)

Gametes

* Sex cells: egg, sperm
* Contain one copy of sex chromosomes
* Contain 22 autosomes and 1 sex chromosome

Heterosomes

Synonym for sex chromosomes

Y Chromosome…

* Contains the genes for developing male sex characteristics 
* The father is always responsible for determining the sex of the offspring:
* If the male sperm contains an X chromosome, the growing embryo will develop into a girl
* If the male sperm contains a Y chromosome, the growing embryo will develop into a boy

Female egg…

In all cases contains an X chromosome (as the mother is XX) 

Karyotype

The number and types of chromosomes in an organism

Karyogram

Visual representation of chromosomes in an organism, chromosomes are photographed and stained to form this

Karyotyping is used to…

* Determine the gender of the unborn child (via identification of the sex chromosomes)
* Test for chromosomal abnormalities (e.g. aneuploidies or translocations)

Down Syndrome

* Condition where the individual has three copies of chromosome 21
* It is caused by the failure of chromosomes to separate in one of the parental gametes
* The extra genetic material causes mental and physical delays in the way the child develops

Vitro Autoradiography

* Method used to visualise chromosomes with photographs
* Better method of measuring chromosome length than using condensed chromosomes during mitosis (due to supercoiling)

Organisms with different diploid numbers…

are less likely to be able to reproduce (can’t form homologous pairs in zygotes

* In cases where different species do interbreed, offspring are usually infertile (cannot form functional gametes)

Chromosome number does not…

provide a valid indication of genetic complexity

* Tomatoes have 24 chromosomes and possess around 32,000 genes
* Chickens have 78 chromosomes but possess only around 17,000 genes

Genome size does not…

provide a valid indication of genetic complexity

Viruses’ and bacteria’s genomes…

tend to be very small

Prokaryotes’ genomes…

tend to be smaller than eukaryotes’ genomes

Mutagen

Something causing a mutation, can be a substance or radiation, for example

Meiosis

Process by which sex cells (gametes) are made in the reproductive organs